Devecer, Hungary (CNN) -- Workers in Hungary were racing Tuesday to build three emergency dams to stem an expected second toxic spill from an aluminum plant.

Some 500,000 cubic meters of toxic red sludge is in the plant's reservoir, whose wall shows signs of ruptures and cracks, said Gyorgi Tottos, a spokeswoman with Hungary's emergency services department.

Officials say it's only a matter of time before the wall breaks and spews the sludge across the landscape, she said.

European Commission President Jose Manuel Barroso was in Hungary on Tuesday for a meeting with Hungarian Prime Minister Viktor Orban. The spill was one topic on their agenda and Barroso reiterated the European Union's promise of assistance and help, Barroso spokesman Cezary Lewanowicz told CNN.

The amount of sludge that remains in the reservoir is about half the amount that spilled out a week ago, inundating three villages, killing eight people, and leaving the landscape covered in red.

It wasn't clear when the emergency dams would be finished. Officials had said the dams would be finished by Tuesday morning, but later they pushed it back to Wednesday morning.

Crews were also trying to remove a layer of liquid from the top of the sludge in the reservoir in order to make the mud less mobile if the wall breaks.

Monday, the head of the company that owns the plant was arrested, accused of public endangerment and harming the environment, authorities said. Orban said he favored strong government intervention against the company, MAL Co.

The company said in a statement Saturday it was doing its utmost "to avoid further damages and to reinforce the injured deposit." The company said it has performed extensive maintenance work and renovations in the past decade and had followed safety regulations.

The company was working to construct dams and defense lines in an attempt to minimize damage, it said. It has also established a relief fund for victims of the spill and was attempting to help in finding accommodations for evacuated people.

About 800 people have been evacuated from the village of Kolontar, downstream from the reservoir, and hundreds of soldiers were ready to rescue inhabitants of a nearby village if the wall collapses.

The sludge from last Monday's spill reached the Danube -- Europe's second-largest river -- on Thursday, raising fears of wider contamination in Hungary and countries downstream, including Croatia, Serbia, Bulgaria, Romania and the Ukraine.

Water test results released Friday, however, indicated the sludge may not harm the Danube after all. Tottos said the pH level of the Danube water was only slightly higher than normal.

The exact chemical composition of the sludge has not been revealed, but aluminum processing normally involves compounds that include cyanide, cadmium and chromium.

Emergency workers have been pouring plaster and fertilizers on the sludge in hopes that it will counter its alkalinity. Dust from sludge that dries may also pose a threat, Hungary's Interior Ministry has said.

CNN's Diana Magnay in Devecer, Hungary, and Melissa Gray in London, England, contributed to this report.